Single-Molecule Magnets:  A New Class of Tetranuclear Manganese Magnets

Abstract

The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn4(O2CMe)2(pdmH)6](ClO4)2 (1). Complex 1.2MeCN.Et2O crystallizes in the triclinic space group P1, with cell dimensions at 130 K of a = 11.914(3) A, b = 15.347(4) A, c = 9.660(3) A, alpha = 104.58(1) degree, beta = 93.42(1) degree, gamma = 106.06(1) degree, and Z = 1. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valent, MnIII2MnII2. The pdmH- ligands (pdmH2 is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1.2.5H2O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1.2.5H2O and indicate that the desolvated complex has a S = 8 ground state, whereas the hydrated 1.2.5H2O has a S = 9 ground state. Ferromagnetic interactions between MnIII-MnII and MnIII-MnIII pairs result in parallel spin alignments of the S = 5/2 MnII and S = 2 MnIII ions. High-frequency EPR spectra were run for complex 1.2.5H2O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99, D = -0.451 K, and B4 degrees = 2.94 x 10(-5) K for the S = 9 ground state of 1.2.5H2O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals (chi'' M) were seen for complexes 1 and 1.2.5H2O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in chi'' M vs temperature plots was seen for each frequency; the temperature of the chi'' M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S = 8 complex with D = -0.35 K. The 1.2.5H2O complex with an S = 9 ground state has its chi'' M peaks at higher temperatures.

Spin-Spin Cross Relaxation in Single-Molecule Magnets

“…Each molecule functions as a nanoscale, single-domain magnetic particle that, below its blocking temperature, exhibits the classical macroscale property of a magnet, namely magnetization hysteresis. Such a molecule straddles the classical/quantum interface in also displaying quantum tunneling of magnetization [4,5,6,7,8,9,10] and quantum phase interference [11,12,13]. A quantitative understanding of the mechanism of magnetization tunneling is being developed.…”

High Spin Mn Molecular Clusters: Spin State Effects On the Outer Core-Level Multiplet Structures

“…As previously stated, the excitation energy was 1253.6 eV, and thus the positions and relative intensities of the Mn 3s and Mn 3p multiplets can be considered to be in the sudden limit approximation [1,2] with little coupling between the ion and the photoelectron. The figure shows a chemical shift of the Mn 3 pi,2,1/2 spin-orbit pair associated with the trapped-valence oxidation states.…”

“…[1,2] These molecules have a large ground state spin S and a large magnetic hysteresis comparable to that observed in hard magnets.…”

First principles study of electronic structure for cubane-like and ring-shaped structures of M4O4, M4S4 clusters (M = Mn, Fe, Co, Ni, Cu)

“…36 Furthermore, the cubane structure allows a significant magnetic exchange between metal ions and therefore, the cubanes containing manganese atoms, in particular, have been studied extensively for their magnetic properties, specially in the search for single molecular magnets. [37][38][39][40][41] On the other hand, planar structure of the M 4 X 4 units usually constitute building blocks for the structures of several layered perovskite oxides as well as cubic perovskite oxides, which have been studied extensively in recent time particularly for searching novel photo-catalysts of hydrogen production from water using the sun light. [42][43][44] In the present work using first principles density functional theory (DFT) based electronic structure calculations, we have performed a relative structural stability of the isolated M 4 X 4 clusters between the two morphologies, as a first step towards understanding distinctly their role and the effects of the surrounding environment in the real systems of poly-nuclear transition metal complexes.…”